© 2016. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2016) 219, 597-608 doi:10.1242/jeb.122275 RESEARCH ARTICLE Spectral sensitivity, spatial resolution and temporal resolution and their implications for conspecific signalling in cleaner shrimp Eleanor M. Caves1,*, Tamara M. Frank2 and Sönke Johnsen1 ABSTRACT or spatial resolution are rare (but see Baldwin and Johnsen, 2011; Cleaner shrimp (Decapoda) regularly interact with conspecifics Johnsen, 2006; Marshall, 2000). and client reef fish, both of which appear colourful and finely Broadly, visual physiology may be adapted for certain tasks, such patterned to human observers. However, whether cleaner as the detection of relevant visual signals (summarized in Cronin shrimp can perceive the colour patterns of conspecifics and et al., 2014; Land and Nilsson, 2002). For example, the multiple clients is unknown, because cleaner shrimp visual capabilities chromatic channels in some primates assist in foraging for fruit are unstudied. We quantified spectral sensitivity and temporal against a leafy background (Osorio and Vorobyev, 1996; Regan ‘ ’ resolution using electroretinography (ERG), and spatial resolution et al., 2001), and acute zones in the eyes of Polistes paper wasps using both morphological (inter-ommatidial angle) and behavioural allow individuals to resolve the facial patterns of conspecifics (optomotor) methods in three cleaner shrimp species: Lysmata (Sheehan et al., 2014). However, existing work on the relationship amboinensis, Ancylomenes pedersoni and Urocaridella between visual capabilities and visual signals has focused on how a antonbruunii. In all three species, we found strong evidence for single aspect of vision relates to the detection of a signal. In reality, ’ only a single spectral sensitivity peak of (mean±s.e.m.) 518±5, 518 multiple aspects of an animal s visual physiology, including ±2 and 533±3 nm, respectively. Temporal resolution in dark- spectral sensitivity and both temporal and spatial resolution, all adapted eyes was 39±1.3, 36±0.6 and 34±1.3 Hz. Spatial play a role in whether or not an animal can detect a given signal. resolution was 9.9±0.3, 8.3±0.1 and 11±0.5 deg, respectively, The interactions of cleaner shrimp with client reef fish are well which is low compared with other compound eyes of similar size. described (reviewed in Côté, 2000; Losey et al., 1999a), but their Assuming monochromacy, we present approximations of cleaner visual ecology is poorly understood. Cleaner shrimp attract clients shrimp perception of both conspecifics and clients, and show that to cleaning stations, which they usually occupy in groups or pairs cleaner shrimp visual capabilities are sufficient to detect the (Bauer, 2006; Briones-Fourzán et al., 2012; Bshary et al., 2007; outlines of large stimuli, but not to detect the colour patterns of Chapuis and Bshary, 2010; Fletcher et al., 1995; Huebner and conspecifics or clients, even over short distances. Thus, Chadwick, 2012b), and then provide cleaning services to clients by conspecific viewers have probably not played a role in the removing ectoparasites, in turn receiving a meal. Thus, cleaner evolution of cleaner shrimp appearance; rather, further studies shrimp interact on a regular basis with conspecifics and client fish, should investigate whether cleaner shrimp colour patterns have both of which, to human observers, display brilliant colours and fine evolved to be viewed by client reef fish, many of which possess tri- patterns. Evidence from the cleaner shrimps Lysmata debelius and and tetra-chromatic colour vision and relatively high spatial acuity. Lysmata amboinensis suggests that the formation and maintenance of conspecific groups relies at least in part on visual cues (Fletcher KEY WORDS: Visual signals, Decapod crustaceans, Colour patterns, et al., 1995; Rufino and Jones, 2001; Simoes and Jones, 1999), Perception, Crustacean vision which could include colour patterns as an intraspecific signal. Additionally, to initiate cleaning, clients approach cleaners and INTRODUCTION adopt a characteristic solicitation pose, a stationary position with Animal visual capabilities are diverse, and understanding how an opercula and/or fins flared out, occasionally accompanied by a organism perceives visual scenes requires knowledge of that colour change (Côté et al., 1998; Hobson, 1971; Huebner and animal’s spectral sensitivity (wavelength sensitivity of the Chadwick, 2012a; Losey, 1972; Stummer et al., 2004); therefore, photoreceptors), temporal resolution (ability to visually track client colour patterns may serve as an interspecific signal. Here, we rapid changes in a scene; Frank, 1999, 2003) and spatial quantified cleaner shrimp visual capabilities to ask whether cleaner resolution (ability to perceive detail). Numerous studies have shrimp can reliably perceive the colours and patterns of conspecifics shown that it is inaccurate to use human perception of colour and and/or clients. luminance to assess how an animal perceives a visual scene (Bennett We examined the visual physiology of three cleaner shrimp et al., 1994; Safran and Vitousek, 2008); however, studies that species: Lysmata amboinensis, Ancylomenes pedersoni and quantify and incorporate receiver-appropriate measures of temporal Urocaridella antonbruunii. To quantify spectral sensitivity and temporal resolution, we used electroretinography (ERG). ERG measures the physiological response of photoreceptors in situ, and is the method of choice for comparative studies, and for examining 1Department of Biology, Box 90338, Duke University, Durham, NC 27708, USA. 2Halmos College of Natural Sciences and Oceanography, Department of Marine spectral sensitivities in animals whose sensitivities have not been and Environmental Sciences, Nova Southeast University, 8000 North Ocean Drive, previously quantified. Crustacean electroretinograms are Dania Beach, FL 33004, USA. monophasic; thus, the ERG response mirrors the response of the *Author for correspondence ([email protected]) photoreceptors alone (Chapman and Lall, 1967; Goldsmith and Fernandez, 1968). To quantify spatial resolution, we used both Received 13 March 2015; Accepted 3 December 2015 morphological and behavioural methods. We then used those visual Journal of Experimental Biology 597 RESEARCH ARTICLE Journal of Experimental Biology (2016) 219, 597-608 doi:10.1242/jeb.122275 (N=4–6 individuals/species). During trials, a plastic post was List of symbols and abbreviations attached to the dorsal surface of the animal using cyanoacrylate glue CFFmax maximum critical flicker fusion frequency (Super Glue, Pacer Technology, Rancho Cucamonga, CA, USA), ERG electroretinography and the animal was suspended in seawater, which was maintained at I irradiance room temperature (approximately 23°C). The animal was placed k irradiance required to generate a response of 50% Vmax such that its eyes were out of the water, but the rest of the body V voltage was submerged, leaving the pleopods free to move and circulate Vmax maximum voltage the eye is capable of producing, maximum electrical response created by the photoreceptors water. α min minimum resolvable angle, a measure of spatial resolution A tungsten microelectrode (FHC Inc., Bowdoin, ME, USA) was ΔΦ interommatidial angle, the separation between adjacent inserted beneath the cornea of the left eye under dim red light ommatidial axes (610 nm cut-off filter), which minimized light adaptation of the eye. This electrode was used to record electrical responses to light stimuli; a differential reference electrode was placed in the parameters to provide a general sense of how cleaner shrimp might surrounding seawater, which was grounded by an AgCl-coated view the patterns and colours of conspecifics and client fish. wire. The eye was exposed to light stimuli, and AC recordings of the These three cleaner shrimp species were selected because they are eye’s response were amplified, digitized, and stored using a custom known fish cleaners (Becker et al., 2005; Chen and Huang, 2012; program written in LabView (National Instruments, Austin, TX, Côté, 2000), are colourful and finely patterned, operate cleaning USA). Cut-off frequencies were 1 Hz and 15 KHz. We used a stations with conspecifics, and live on shallow coral reefs (often Spectral Products (Putnam, CT, USA) CM110 monochromator <20 m deep), which are spectrally broad, high light intensity (175 W xenon lamp) to shine a stimulus light of controlled environments. We assumed that cleaner shrimp, which display a wavelength on the eye of the animal, via one branch of a bifurcated, variety of colours, have colour vision. Additionally, based on the randomized fibre light guide (EXFO, Quebec City, QC, Canada), high light intensity environment in which they live and their regular which was placed so as to bathe the entire eye in a diffuse circle of interactions with finely patterned conspecifics and client fish, we light (rather than exposing the eye to a pinpoint source). Bathing the predicted that cleaner shrimp have high temporal resolution and fine entire eye in light allowed as many ommatidia as possible to spatial acuity that would allow them to perceive conspecific and contribute to the response, and maximized the likelihood that we client colour patterns over ecologically relevant distances. would be able to detect any secondary receptor types if present. A computer-controlled shutter (model VS25, Uniblitz, Rochester, MATERIALS AND METHODS NY, USA) set the stimulus duration. The irradiance